Unfortunately as far as I'm aware none of the ASUS GT routers are supported by any third party firmware. If you want something close that does work look at the ASUS RT-AC86U or similar non-GT model.
Just bought GT-AC5300 on advice that by installing asuswrt firmware I could configure it as a VPN router by creating a NordVPN VPN Client connection like I have on my RT-AC87U which works perfectly.
Tried loads of different versions and every time I try to upload I get "Invalid Firmware Upload", so does the router actually support merlin? - if so, has Asus stopped it in some way and is there a workaround? if not it's back to Amazon!
thanks in advance